The present invention relates generally to the field of semiconductors. More specifically, the present invention relates to multilayer high dielectric constant (xcexa) films and methods of making such films.
Design and manufacturing of integrated circuits (ICs) are becoming increasingly complex as the device density of such circuits increases. High density circuits require closely spaced devices and interconnect lines, as well as multiple layers of materials and structures. FIG. 1 schematically shows a conventional metal oxide semiconductor field effect transistor (MOSFET) device that consists of a gate, a gate dielectric such as silicon dioxide (SiO2), and a source/drain channel region. As the size of IC device geometry becomes aggressively smaller, the thickness (t) of the gate dielectric, in the case of SiO2, reaches a physical limitation of approximately 20 Angstroms. Below this thickness, the conventional SiO2 gate dielectric no longer functions as an insulator due to direct tunneling of electrons between the gate and the channel region. Thus, SiO2 gate dielectrics are rapidly becoming one of the limiting factors in device design and manufacturing.
To address this problem, alternative gate dielectric materials have recently been investigated. One approach is to replace the SiO2 gate dielectric with a material that has a higher dielectric constant (xcexa) than SiO2 (xcexa of SiO2 is approximately 3.9). It has been found that when a material with a high dielectric constant is used, the physical thickness (t) of the gate dielectric can be increased while maintaining its gate capacitance. Namely, physically thicker high xcexa gate dielectric can have electrical properties as good as or better than thin SiO2 gate dielectric. A thicker film is easier to manufacture than a thin film and may exhibit better electrical properties.
In the prior art, a number of different high xcexa materials have been developed. For example, TiO2, Ta2O5, BaxSrixe2x88x92xTiO3, ZrOx and HfOx have been experimented with as gate dielectrics. However, these materials are subject to limitations. TiO2 and Ta2O5 are thermally unstable and tend to form an undesirable layer of silicon oxide at the interface of the silicon substrate and the gate dielectric. BaxSrixe2x88x92xTiO3 films require high temperature processing which make them undesirable for device integration. While ZrOx and HfOx have a higher dielectric constant which is about 25, ZrOx and HfOx alone are not suitable as gate dielectrics since undesirable silicon oxide is also formed at the interface between the silicon substrate and the gate dielectric. The formation of additional silicon oxides increases the equivalent oxide thickness (EOT) of the gate dielectric which will result in degradation of device performance.
More recently, zirconium silicate (ZrSixOy) and hafnium silicate (HfSixOy) have been investigated as new gate dielectric materials. For example, zirconium silicate gate dielectrics have been reported in xe2x80x9cStable zirconiumii silicate gate dielectrics deposited directly on siliconxe2x80x9d by G. D. Wilk and R. M. Wallace, Applied Physics Letters, Volume 76, Number 1, Jan. 3, 2000 pp. 112-114 and in xe2x80x9cElectrical properties of hafnium silicate gate dielectrics deposited directly on siliconxe2x80x9d by G. D. Wilk et al., Applied Physics Letters, Volume 74, 1999 pp. 2854-2856. Zirconium silicate and Hafnium silicate are of particular interest as an alternative gate dielectric material because of its relatively high dielectric constant value. Its dielectric constant is marginally increased to about 10 to 15 depending upon the ZrOx/HfOx content in the film. Moreover, zirconium or hafnium silicate exhibits thermal stability in direct contact with the silicon substrate. However, the dielectric constant is not as high as seen with other materials, and such films have not been successfully employed in commercial operation. Accordingly, a significant need exists for the development of high dielectric constant films.
Accordingly, it is an object of the present invention to provide a high dielectric constant (xcexa) films for gate dielectrics.
It is further an object of the present invention to provide a semiconductor transistor that incorporates the high xcexa dielectric film as the gate dielectric.
Another object of the present invention to provide a method of making a high xcexa dielectric film.
These and other objects are achieved by a new multilayer dielectric film of the present invention employing metal silicates on a silicon substrate and metal oxides having high xcexa to enhance the performance of semiconductor transistors.
In accordance with the present invention, there is provided a multilayer dielectric film that comprises a first layer formed of a material having a high dielectric constant, and a second layer formed on the first layer. The second layer is formed of a material having a dielectric constant lower than the dielectric constant of the first layer. The first layer is preferably comprised of a metal oxide material having a dielectric constant in the range of 15 to 200, and the second layer is preferably comprised of a metal silicate material having a dielectric constant in the range of 5 to 100.
In one preferred embodiment, the multilayer dielectric film of the present invention comprises a first layer of a metal oxide having the formula of MxOy, and a second layer of a metal silicate having the formula of MxSiOy, where M is a metal independently selected from the group consisting of Zr, Hf, Ti, V, Nb, Ta, Cr, Mo, W, Mn, Zn, Al, Ga, In, Ge, Sr, Pb, Sb, Bi, Sc, Y, La, Be, Mg, Ca, Sr, Ba, Th, Lanthanides (Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), and mixtures thereof, x is a number in the range of 1 to 3, and y is a number in the range of 2 to 5. Each of the metal oxide first layer and metal silicate second layer may contain more than one metal elements. The metal in the first and second layers can be same and/or different.
In another preferred embodiment, the multilayer dielectric film of the present invention comprises a first layer of a metal oxide selected from the group consisting of ZrO2 and HfO2, and a second layer of a metal silicate selected from the group consisting of Zrxe2x80x94Sixe2x80x94O and Hfxe2x80x94Sixe2x80x94O.
In another embodiment of the present invention, the multilayer dielectric film of the present invention comprises a first layer having a first and second surfaces, a second layer formed on the first surface of the first layer, and a third layer formed on the second surface of the first layer, wherein the second and third layers are comprised of a material having a dielectric constant lower than the dielectric constant of the first layer.
In another aspect of the present invention, there is provided a method of forming a multilayer dielectric film on a substrate. The method comprises the steps of forming a metal silicate layer on the surface of a substrate, and forming a metal oxide layer atop the metal silicate layer. In one embodiment, the method further comprises forming another metal silicate layer atop the metal oxide layer. The forming step can be carried out by chemical vapor deposition, physical vapor deposition, atomic layer, deposition, aerosol pyrolysis, spray coating or spin-on-coating.